Hossein Masoumi

Unified Size-Effect Law for Intact Rock

AbstractA suite of laboratory testing was performed on Gosford sandstone samples having a range of sizes, including point-load and uniaxial compressive tests. A unified size-effect law (USEL), based on the work by Zdenek Bazant, involving fracture energy as well as fractal theories, was introduced. It was shown that USEL correlates well with the ascending and descending uniaxial compressive strength trends obtained from Gosford sandstone as well as five other rock types reported by Brian Hawkins. Fractal characteristics found to be the primary mechanism for ascending strength trends and surface flaws could be considered as a secondary mechanism. The influence of the contact area on the size-effect behavior of point-load results was investigated using a new approach. This approach was novel in the way it incorporated the load contact area. Determination of the point-load strength index using this new approach led to opposite size-effect trends compared with those observed using a conventional point-load st...

Microstructural effects on mechanical properties of shaly sandstone

AbstractUnderstanding the mechanical properties of shaly sandstone is of great importance in reservoir geomechanics. Because of the lack of core data, measurements based on acoustic wave velocities...

Size-Dependent Hoek-Brown Failure Criterion

AbstractThe size dependency of intact rock is of importance to different disciplines, such as civil and mining engineering. One example relates to the design of structures on or within a rock mass for which an estimation of the strength of the intact rock blocks within the mass is essential. Despite a large number of studies on size effects in rock, less research has investigated size effect under triaxial conditions. Thus, a suite of advanced triaxial compressive experiments was conducted on Gosford sandstone samples with diameters of 96, 50, and 25 mm. A size-dependent Hoek-Brown failure criterion was developed by incorporating a unified size-effect law into the original Hoek-Brown failure criterion. The model was calibrated against the triaxial data obtained from Gosford sandstone. It was shown that there is good agreement between the proposed model prediction and the experimental results. Finally, an example of application of the size-dependent Hoek-Brown failure criterion was presented to demonstrate...

Modification to Radial Strain Calculation in Rock Testing

The complete uniaxial or triaxial stress–strain behavior of a rock sample can be achieved in compressive testing using a servo-controlled testing system. Initially, the load-deformation data is recorded during an experiment, and then to eliminate the scale dependency of force, this data are converted to the stress–strain curve for final reporting. It has been found that because of the particular design of the current sample instrumentation system used to measure the circumferential or lateral deformation during the uniaxial or triaxial compressive tests, a modification is required to be included in the conventional method of radial strain calculation. As a result, a new analytical solution has been developed to improve the accuracy of the radial strain calculation. The modification procedure differs depending on whether the test-sample diameter is greater or less than 50 mm. A comparative study was conducted based on visual observation and quantitative analysis to show the variation in a number of rock parameters including elastic modulus, Poissons ratio, peak stress, critical strain, residual stress strain, softening fracture energy, and residual stress when this modification was implemented. For a 50-mm-diameter sample, which is the suggested size for rock testing according to the International Society for Rock Mechanics, the effect of this modification on the parameters obtained from the post-peak region, such as critical strain, residual stress strain, and softening fracture energy varied from 5 % to 9 %. It was concluded that the impact of this modification procedure is more significant with smaller-diameter samples used in rock testing.

Establishing Empirical Relationships for the Effects of Water Content on the Mechanical Behavior of Gosford Sandstone

List of symbols UCS Uniaxial compressive strength E Young’s modulus PLI Point-load index TS Tensile strength x Water content Mps Mass of rock sample under partially saturated condition Md Mass of dry rock sample r Radius of the sample H Hydraulic diffusivity k Permeability B Skempton’s coefficient K Bulk modulus g Fluid viscosity a Biot’s coefficient P Maximum applied load measured during the point loading D Distance between two pointers (conical platens) in diametral point-load test which is also the core sample diameter L Length of the core sample F Peak load measured during the Brazilian or indirect tensile test T Thickness of the sample measured at the center rn Nominal strength such as UCS, E, PLI, TS a, b and c Material constants in Hawkins and McConnell (1992) model

Mechanical Characterisation of Modified Cable Bolts Under Axial Loading: An Extensive Parametric Study

A wide range of modified cable bolts are currently used for ground support in different conditions in Australian underground coal mining operations. This is mainly based on the belief that the performance of modified cable bolts is better than conventional cable bolts. Despite this positive view, very few studies have characterised the performance of modified cable bolts in service. A new laboratory-based pull-out testing facility was employed in this study to investigate the behaviour of two different types of modified cable bolts under axial loading including Garford twin strand bulbed and MW9P (smooth wire). The impact of different parameters including the compressive strength of the grout and the confining medium as well as borehole diameter on the performance of Garford twin strand bulbed cable bolt was investigated. In the case of the MW9P cable bolt, only the effects of borehole diameter and grout compressive strength were examined. A full factorial experimental programme was designed based on Taguchi method to explore the performance of these two cable bolts under different testing conditions. Consequently, an analysis of variance (ANOVA) was conducted to assess the weighting contribution of each parameter (e.g. borehole diameter, the compressive strength of grout and confining medium) to the load–displacement performance of Garford twin strand bulbed and MW9P cable bolts. For the Garford twin strand bulbed cable bolt, it was concluded that the compressive strength of the confining medium was the most influential parameter in the determination of the peak and residual loads as well as initial stiffness. It was also observed that an increase in the compressive strength of either the confining medium or the grout led to an increase in peak and residual loads in the Garford twin strand bulbed cable bolt, while borehole diameter had no sensible effect on peak and residual loads as well as initial stiffness. In the case of the MW9P cable bolt, the compressive strength of grout had the greatest impact on the peak and residual loads whereby, the peak and residual loads varied directly with grout compressive strength. A 10-mm increase in borehole diameter of MW9P cable bolt had negligible effect on the peak and residual loads as well as initial stiffness.